GaN-based Heterojunction Field Effect Transistors (HFETs) using a wide bandgap semiconductor such as GaN, AlGaN, InGaN, AlGaN, InAlN, AlInGaN and the like have received much attention as a power device for high power application since they are one order of magnitude or more smaller in on-resistance than transistors using Si or GaAs, are capable of operating at higher temperature with high current and can withstand high voltage applications.
One example of a conventional GaN-based HFET is shown in FIG. 1. IN FIG. 1, as well as the figures that follow, like reference numerals are used to denote like elements. As shown, a heterojunction structure is formed on a substrate such as a sapphire, silicon, or diamond substrate 91. The heterojunction structure includes a nucleation layer 92 of GaN, for example, a semi-insulating undoped GaN layer 93, and an undoped AlGaN layer 94, which is generally much thinner than the undoped GaN layer 93. The two-dimensional (2-D) electron gas channel 79 is located adjacent to the interface of layers 93 and 94. An optional layer of GaN a few atomic layers thick may optionally be formed on top of AlGaN layer 94, which is not shown in FIG. 1 or the figures that follow. The optional GaN layer may improve the ohmic contact and often is removed naturally after ohmic annealing and chemical treatment. Two n-AlGaN/GaN contact layers 95 are disposed on the undoped AlGaN layer 94 and annealed to form ohmic contact with the 2-D gas. A source electrode S and a drain electrode D are arranged on their respective contact layers 95. A gate electrode G is formed onto the undoped AlGaN layer 94 and is situated between the source electrode S and the drain electrode D.
The GaN-based HFET device is capable of maximizing electron mobility by forming a quantum well at the heterojunction interface between the AlGaN layer, which has a larger band gap, and the GaN layer, which has a narrower band gap. As a result, electrons are trapped in the quantum well. The trapped electrons are represented by a two-dimensional electron gas 79 in the undoped GaN layer. The amount of current is controlled by applying voltage to the gate electrode, which is in Schottky contact with the semiconductors so that electrons flow along the channel between the source electrode and the drain electrode.
Even when the gate voltage is zero, electrons will be present in the channel because a piezoelectric field is formed that extends from the substrate toward the device surface. Consequently, the GaN-based HFET acts as a depletion-mode (i.e., normally-on) device. For a variety of reasons it would be desirable to provide an enhancement mode (i.e., normally-off) GaN-based HFET. For example, when a depletion-mode HFET is employed as a switching device for a power source, it is necessary to continuously apply a bias voltage to the gate electrode that is lower than the threshold value to keep the switch in the off state. Such an arrangement often results in a more complicated circuit and requires more voltage levels such as negative voltages. On the other hand, if an enhancement mode HFET is employed, the switch can be maintained in the off state even a gate voltage of zero using a simplified circuit. Attempts have been made to manufacture GaN-based enhancement-mode HFETs by depleting or eliminating the two-dimensional gas in the gate area. Unfortunately, such attempts have generally not been satisfactory when pursuing a high positive threshold voltage because of problems such as poor on-state currents, poor breakdown voltages and lower operational speeds due to the resistance and capacitance in the gate regions as well as damage that is caused to the gate regions during fabrication. Therefore, there is an inherent trade-off between a high threshold voltage and a high on-state current/speed.